Abort previous build targets when a build is restarted

Summary: Ref T5936. This implements build implementations aborting early when the build has since been restarted. Build steps now periodically poll to see if the build's current generation does not match their generation, and they throw a HarbormasterBuildAbortedException if that is the case.

Test Plan: Tested locally on my machine with the sleep build step.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T5936

Differential Revision: https://secure.phabricator.com/D10322